The present invention relates to single-stage or multi-stage centrifugal fluid conveying machines in general, and more particularly to improvements in single-stage or multi-stage blowers or fans for gaseous and/or other fluids which are aggressive because of their elevated temperature and/or for other reasons. Still more particularly, the invention relates to improvements in centrifugal blowers or fans (hereinafter called centrifugal machines) which are designed to normally convey large quantities of gaseous and/or other fluids per unit of time.
It is already known to provide the housing of the rotor of a centrifugal machine with radially inwardly extending vanes which draw a gaseous fluid from a first pipe and deliver the fluid into a second pipe when the rotor is driven by a motor or the like. As a rule, the rotor is a body of rotation and its vanes are made of a material which is capable of standing elevated temperatures and/or the chemical action of a hot and/or otherwise aggressive fluid. For example, German Utility Model No. 7,029,967 discloses a gas turbine which defines a bell-shaped combustion chamber and whose rotor has a hollow housing as well as vanes which extend radially inwardly from the housing. The exterior of the bell-shaped combustion chamber is cooled by air streams. A drawback of such machines is that their output is relatively low if the conveyed fluid is maintained at an elevated temperature and such fluid must undergo at least some or pronounced compression during flow through the machine. The reason is that the resistance which a heat-resistant or chemically resistant material offers to bending and/or tensional stresses decreases very rapidly with increasing temperature of the conveyed fluids, even in response to heating to a relatively low temperature. This applies, for example, to the materials for use in machines that convey very hot gases which are circulated or otherwise conveyed in connection with research involving coal. Thus, if the temperature of such gases rises to approximately 800.degree. C., the stability of the material of heretofore known conveying machines decreases very rapidly so that the RPM of the machines has to be drastically reduced, even if the material of such machines is a high-quality steel. Therefore, a single-stage blower which serves as a means for conveying such gases is incapable of raising the pressure of conveyed gases above 150 mm water column. In fact, such machines are incapable of conveying gases or molten metals whose temperature is in the range of 1200.degree. C., not to speak of temperatures as high as 1600.degree. C. External cooling with streams of atmospheric air cannot furnish the required cooling action when the temperatures rise above 800.degree. C. and approach or exceed 1200.degree. C., especially if the machine is to be operated at a high RPM in order to achieve the requisite throughput and/or compression. While these conventional machines could be operated under the above outlined circumstances by resort to pronounced cooling with streams of air whose temperature is well below room temperature, the cost of cooling would be prohibitive because the energy requirements of the cooling system would render the operation utterly uneconomical.
British Pat. No. 867,716 discloses a gas turbine wherein the material of the vanes and of a ring which surrounds the vanes is a ceramic substance. Such material can stand elevated temperatures; however, it is a good conductor of heat and is incapable of standing even average tensional stresses. Therefore, the turbine of this British patent is provided with an annular plenum chamber which surrounds the aforementioned ring and wherein the pressure of a cooling gas is sufficiently high to partially or completely neutralize the action of centrifugal forces upon the rotor ring and rotor blades. It has been found that the patented turbine presents numerous and serious problems as regards the establishment of seals between stationary and rotating parts and also as concerns the withdrawal of heat from the fluid which fills the plenum chamber. Such heat is transmitted by the ceramic components of the rotor. Withdrawal of heat from the plenum chamber necessitates the provision of a cooling system which is so expensive and whose energy requirements are so high that the patented turbine is utterly uneconomical for a majority of applications. The situation is aggravated if the fluid in the plenum chamber must be maintained at an elevated pressure, i.e., if the action of centrifugal forces upon the ceramic components of the rotor is very pronounced. Consequently, for all practical purposes, the patented turbine is capable of operating only within a relatively low RPM range which is insufficient to allow for adequate compression of certain fluids and/or for conveying of such fluids at the required rate.
All in all, the aforedescribed conventional centrifugal fluid conveying machines and analogous machines are either incapable of conveying very hot and/or otherwise aggressive fluids at the required rate and/or pressure, or are so expensive that they cannot be used under a majority of circumstances.